The Mayor’s office has warned that people should stay indoors when possible, not allow children outside unnecessarily and avoid undertaking any strenuous activity outside.

In a statement the Mayor said:

“Today the shameful state of London’s toxic air has meant that I am forced to trigger the first ‘very high’ air pollution alert under my new comprehensive alert system.

“This is the highest level of alert and everyone - from the most vulnerable to the physically fit - may need to take precautions to protect themselves from the filthy air.

London’s second day of extremely high air pollution comes just several months after the World Health Organisation revealed that more than 80 per cent of the world’s population living in urban areas are being exposed to extreme levels of air pollution.

The figures come from a database that now comprises over 3000 cities spanning 103 countries.

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Despite this damning evaluation of life in the city, many countries are still struggling to find a long-term solution to reduce the amount of pollution that's being generated.

Air pollution is one of the greatest causes for early death in the world, with indoor pollution alone contributing to 99,000 deaths in Europe.

What's perhaps most staggering is that 10,000 of those people live in London.

So what is it that air pollution is actually doing to us? Because aside from the visible signs of smog it can be difficult to truly understand the health dangers that come with inhaling pollutants.

What does air pollution actually do to you?

According to DEFRA air pollution mainly (and perhaps unsurprisingly) affects the respiratory and inflammatory systems.

It lists the most common pollutants as Nitrogen Dioxide, Sulphur Dioxide, Ozone, Particles and Carbon Monoxide.

The first three are known to irritate the airways of the lungs, increasing the symptoms of those with lung diseases.

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Particles meanwhile are carried deep into the lungs and can cause severe inflammation with lung and heart diseases.

Finally Carbon Monoxide prevents the uptake of oxygen by the blood. This in turn then significantly reduces the amount of oxygen reaching the heart.

As a result of this WHO points out that 80 per cent of outdoor air pollution-related deaths were caused by ischaemic heart disease and strokes, while 14 per cent of deaths were caused by chronic obstructive pulmonary disease or acute lower respiratory infections. Finally, six per cent were due to lung cancer.

While air pollution in London is very unlikely to be the direct cause of these deaths, what it does instead is simply increase the risk of a person developing a cardiovascular and respiratory diseases, as well as of lung cancer.